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The P-T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid-greenstone contact is characterized by a contact-parallel highstrain zone that separates the generally low-grade, greenschist facies greenstone belt from mid-crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600-700°C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of 30-40°C km )1 . The most likely setting for these conditions was a mid-or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid-crustal levels, followed by crustal exhumation related to the late-to post-collisional extension of the granitoidgreenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P-T paths indicate near-isothermal decompression to conditions of 500-650°C and 1-3 kbar, followed by near-isobaric cooling to temperatures below 500°C. In metabasic rock types, this retrograde P-T evolution resulted in the formation of coronitic Ep-Qtz and Act-Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solidstate doming of the footwall gneisses and strain localization in contact-parallel greenschist-facies mylonites that overprint the decompressed basement rocks.

The processes leading to the assembly of supracrustal greenstone sequences and plutonic rocks of the trondhjemite-tonalitegranodiorite (TTG) suite, the two main building blocks of all Archean cratons, form a central aspect of our understanding of early continental growth. Key metamorphic and structural data are presented from granitoidgreenstone contacts in the Steynsdorp dome, the oldest and one of the best preserved parts of the Mesoarchean Barberton granitoidgreenstone in South Africa. Pressure and temperature estimates of P = 10-13 kbar and T = 640-660 °C from supracrustal rocks of the Steynsdorp dome indicate the burial of these rocks to depths >30-40 km during the main phase of collisional (D2) tectonics recorded in the terrane. Peak metamorphic assemblages defi ne fl attening-type fabrics, indicating crustal stacking of cool, rigid crust. The subsequent retrogression of rocks is associated with constrictional fabrics that are interpreted to have formed in response to the orogen-parallel extrusion and exhumation of rocks. The progressive retrogression of rocks and associated fabric development, unidirectional lineation and fold plunges, and consistent granitoid-up, greenstone-down kine matic indicators point to the exhumation of the TTG gneisses along an extensional detachment below the low-grade Barberton greenstone belt. The results are consistent with fi ndings along the granitoid-greenstone contacts some 40 km west of the Steynsdorp area, and they indicate the regional extent of this metamorphic core complex and the allochthonous nature of the high-pressure, low-temperature terrane with respect to the rest of the greenstone belt. This also implies that the 3.45 Ga suite of TTG rocks south of the Barberton greenstone belt is unlikely to have represented the source of similar-aged volcanic rocks in the belt. More voluminous and larger extent of 3.45 Ga TTG plutonism represents a signifi cant early crust-forming event in the Barberton terrain, the evidence of which has been obliterated during later epi sodes of tectonomagmatic recycling. The existence of the high-pressure, lowtemperature terrane in the southern Barberton granitoid-greenstone terrain indicates the presence of a cool and rigid continental nucleus in the Mesoarchean around which plate-tectonic processes could initiate.